The chemistry of the type-specific antigens and lipids which provide the basis for the most critical means of identifying and classifying atypical mycobacteria are being examined. Within the 31 member MAIS serocomplex, encompassing all known strains of Mycobacterium avium, M. intracellulare and M. scrofulaceum, the type-specific antigens and lipids are peptidoglycolipids with the generalized structured Fatty Acyl-Phe-alloThr-Ala-Alaninol-(3,4-Me-Rhamnose) Oligosaccharide. It appears that the Fatty Acyl-Peptidyl-(3,4-Me-Rha) provides an invariable core common to all MAIS serotypes to which is attached a variable oligosaccharide. The oligosaccharides are generally small (maximum size is probably a decasaccharide) and may contain constant and inconstant regions. The features common to the oligosaccharides from several serotypes are the presence 6-deoxytalose as the linkage sugar, and rhamnose, 2,3- and 3,4-Me-Rhamnoses as other constituent sugars. In addition, some serotypes contain their own specific sugars, such as 2-O-methylfucose in the case of serotype 25, and an unidentified sugar X in the case of serotype 9. Proposed research will concentrate on the detailed chemistry of the oligosaccharides from a few select serotypes. In addition, the biosynthesis of these substances will be probed in the hope of providing a locus specific to atypical mycobacteria for chemotherapeutic assault. Other possibilities which will be explored are that these antigens provide a capsule protective against the phagolysosomal environment of the host cell, and that type differences may arise from lysogenic conversion by bacteriophage.